Flow sensors which operate according to a calorimetric principle are used in industrial applications. The calorimetric principle involves heating a thermistor which is a temperature dependent element or resistor, that is exposed to the flow to be monitored. The flow cools the thermistor resulting in a resistance change as a function of the velocity of the flow. This resistance change can be evaluated to determine the flow rate. Since the temperature change of the heated thermistor may be the result of changes in the temperature of the medium as well as the flow, special measures must be taken to compensate for or eliminate the influence of changes in the temperature of the medium. The present invention relates to an improved method and apparatus for measuring flow using calorimetric principles.
Conventional calorimetric type flow monitors use two thermistors, one of which is heated to determine the flow rate, the other of which is unheated and used to measure the temperature of the medium. The thermistors are selected with the same temperature constant, so that the ratio of the two thermistors remains constant regardless of the temperature of the medium so long as the flow rate remains constant. Thus, the ratio of the two thermistors changes only with a change in the flow rate.
A problem arises in the two thermistor designs, particularly when they are arranged in a sensor immersed in a pipeline. If the temperature of the medium whose flow is being measured differs from the ambient temperature of the pipeline, a temperature drop occurs between the medium and the ambient temperature via the sensor housing. Thus the two thermistors should be arranged as closely together as possible so that they are exposed to the same temperature. However, this results in a heat flow from the heated thermistor to the reference unheated thermistor. Thus, there is a temperature difference between the temperature of the medium and the temperature of the reference thermistor. To avoid the heat flow from the heated to the unheated thermistor, the two thermistors should be spaced apart as far as possible which results in a distortion of the required reference temperature due to the temperature gradient between the temperature of the medium and the ambient temperature.
Accordingly, conventional calorimetric type flow sensors are reliable only where the temperature of the medium is the same as the ambient temperature. The greater the difference between the two, the greater the error of the sensor signal. As a result, such devices typically used as flow monitoring devices produce faulty signals where there is a temperature differential between of the medium and the surroundings. In order to compensate for the temperature differential, can be equipped with a bridge circuit where the heated thermistor serving as the flow sensor is connected in series with the unheated thermistor serving as the reference sensor. The bridge circuit provides a large signal yield from the flow sensor but also results in distortion in the reference sensor.
The present invention was developed in order to overcome these and other drawbacks of the prior devices by providing a flow sensor which compensates for the temperature of the medium while providing accurate measurements of the flow thereof.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for measuring the flow of a medium using a thermistor as a temperature sensor arranged in the medium and a heating device for periodically heating the thermistor to a first temperature. Once heated, the thermistor cools and its resistance changes as a function of the flowing medium. The changing resistance is reflected in an output signal from the thermistor. A bridge circuit is connected with the thermistor to maintain a constant voltage as the temperature of the medium changes, thereby to compensate for fluctuations in the temperature of the medium, whereby the output signal is solely a function of the flow of the medium.
According to a further object of the invention, the bridge circuit includes a control device operated by the heating device to prevent compensation of the sensor during periodic heating of the thermistor. The heating device preferably comprises a resistance element arranged in spaced relation from the thermistor and a pulse generator connected with the resistance element for periodically energizing the resistance element to heat the thermistor to the first temperature.
The control device may comprise a transistor, a current or voltage source, an operational amplifier, or a microprocessor.